The present invention relates to spectral imaging. In the past, when it was desired to image more than one linear incident polarization using a polarization sensitive spectral filter, such as an acousto-optical tunable filter, other devices in front of the acousto-optical tunable filter were needed to do that. In the past, one could use an electrically tuned liquid crystal variable retarder (LCVR) before the acousto-optical tunable filter, which enabled spectral images with both the vertical and horizontal incident polarizations of incident light. The retarder lets the vertically polarized light pass as it is but when the horizontally polarized light is incident it applies a phase shift or retardance to change the transmitted polarization to vertical as required by the acousto-optical tunable filter.
However, it is desirable to have an imager containing a polarization sensitive spectral filter detect the scene of interest with four different linear incident polarizations of light at each wavelength band contained in the broad spectral range. In some applications it is desirable to also detect all four linear plus two circular polarizations. Another approach is to use two LCVRs in tandem with the acousto-optical tunable filter (AOTF) or liquid crystal tunable filter (LCTF). These devices operate from 400-1800 nm only and the two LCVR approach performance is not consistent.
To describe the polarization behavior completely using measured intensities with vertical, horizontal, 45° and 135° polarizations, i.e., I0, I90, I45 and I135, respectively, the Stokes polarization parameters S0=I0+I90, S1=I0−I90 and S2=I45−I135 are used. The parameter S0 corresponds to the total intensity, S1 is the polarization difference between vertical and horizontal polarization and S2 is the polarization difference between 45° and 135° polarization data at each pixel in the corresponding image. When circular polarization is also present, a fourth Stokes parameter is defined as S3=IRCP−ILCP, where IRCP and ILCP are measured intensities of light with right circular polarization (RCP) or counterclockwise circular polarization and left circular polarization (LCP) or clockwise circular polarization, respectively. The parameters S1 and S2 are used as measures to clearly show influence of the four linear polarizations in the images while S3 shows the influence of the two circular polarizations. Measurement of image intensities with all six polarizations completely determines all four parameters S0, S1, S2 and S3. The Stokes parameters are useful in describing any polarization state of light—unpolarized, partially polarized and completely polarized.